Integrated circuits made with dual-in-line packages have leads which are soldered to a printed wiring board. The leads provide a degree of flexibility between the package and the printed wiring board and, therefore, thermal mismatches between the packages and the board do not result in the cracking of the board. These systems are accepted by the Department of Defense if they are capable of withstanding thermal cycling from -60.degree. C. to 150.degree. C.
Because the leads provide a longer electrical pathway, dual-in-line packages are not suitable for very high speed processing of information (generally known as VHSIC). In order to shorten the electrical pathway, leadless chip carriers (LCC) have been developed. Leadless chip carriers have pads that are directly soldered onto the printed wiring board, thereby reducing the length of the circuit and increasing the density of the components mounted on the board. The latter consideration is very important for very large seak integration (VLSI). It was found, however, that printed wiring boards made from E-glass (electrical grade glass) fabric impregnated with epoxy or polyimide resins were not thermally compatible with the chip carrier material. If the leadless chip carrier is a ceramic (known as a hermetic chip carrier, HCC), the ceramic has a coefficient of thermal expansion in the XY plane of 6.times.10.sup.-6 /.degree.C. while the laminate has a coefficient of thermal expansion of 12 to 18.times.10.sup.-6 /.degree.C. As a result, during thermal cycling, the solder joint is strained and undergoes a plastic flow. After a number of cycles, the solder joint usually cracks, breaking the electrical circuit.
An attempt was made to solve this problem by manufacturing printed wiring boards using a polyaramid fiber instead of the E-glass fabrics. Polyaramid fibers have the interesting property of having a negative coefficient of thermal expansion in their axial direction of a magnitude of 2.times.10.sup.-6 /.degree.C. It has been found that printed wiring boards made with a woven polyaramid fabric and a polyimide or an epoxy resin have coefficients of thermal expansion of about 6.times.10.sup.-6 /.degree.C., and that solder joint cracking usually does not occur with these boards.
Unfortunately, the solution to one problem always seems to create other problems and, in this case, the use of the polyaramid, while it eliminated solder joint cracking, resulted in microcracking of the laminate itself during the thermal cycling from -60.degree. C. to 150.degree. C., required by the Department of Defense. The microcracking of the laminate cannot be tolerated because the microcracks may themselves break a circuit or they may permit the absorption of moisture, resulting in shorts. Until now, the problem of eliminating microcracking in polyaramid fiber expoxy laminates has not been solved.